1. Field of the Invention
This invention relates to a display device using contrast media which change the reflectivity upon the application of an electric field. In particular, the invention relates to a display device formed by combining a substrate of an organic resin film with thin film transistors (hereinafter referred to as TFTs) and the contrast media, as well as to a method of fabricating the same.
2. Description of the Related Art
Unlike means that uses a cathode-ray tube, a display device that forms a screen on a flat plate is designed to give electric signals to pixels which are arranged in nearly a two-dimensional manner, and to impart brightness and density by utilizing electro-optical properties of liquid crystals or light-emitting properties of electroluminescence medium, thereby to display an image. Attention has now been given to the display device of this kind which is a new type of display medium by using an electrophoretic material filled in microcapsules.
The contrast medium with a form of microcapsule, which can be electronically addressed is also called electronic ink. Referring to FIG. 10, the electronic ink includes microcapsules 706 of a diameter of about 80 μm in which are contained a transparent liquid, fine white particles 701 which are positively charged and fine black particles 702 which are negatively charged. When the microcapsules 706 are placed in an electric field, the fine white particles 701 and the fine black particles 702 migrate in the opposite directions. When a positive or negative electric field is applied between an counter electrode (transparent electrode) 703 and pixel electrodes 704, 705 as shown in FIG. 10, fine white particles or fine black particles appear on the surface to display white or black color, respectively. The electronic ink and the counter electrode (transparent electrode) can be formed by a printing method; i.e., the electronic ink is printed onto a circuit substrate to form a display device.
The display device using the electronic ink has an advantage in that it consumes less electric power than the liquid crystal display device, owing to its reflectivity of about 30% which is several times as much as that of the reflective liquid crystal display devices. Due to its low reflectivity, the reflective liquid crystal display devices are advantageous in places of intense light such as in the sunshine, but needs an auxiliary illumination such as a front light in a place of weak light. On the other hand, the display device using the electronic ink needs no front light owing to its high reflectivity. The front light needs an electric power of several hundreds mW, which, however, is not required by the display device using the electronic ink. Further, when continuously driven on direct current drive, the liquid crystals are deteriorated. Therefore, the liquid crystal displays must be driven in an alternately inversing manner on alternating current drive. When the frequency of inversion is low, however, there appears flickering which is offensive to see. Usually, therefore, the inversion drive is conducted on alternating current drive of from 60 to 100 Hz. The display device using the electronic ink is not required to be driven in an inverting manner on alternating current drive unlike the liquid crystal displays and, hence, there is no need of effecting the writing every time at 60 Hz. The above-mentioned two points make it possible to decrease the consumption of electric power.
In connection with the electronic ink, electrophoretic display devices using, for example, amorphous silicon (a-Si) TFTs have been reported in “12.1 “SVGA Microencapsulated Electrophoretic Active Matrix Display for Information Appliances SID01 DIGEST pp. 152–155” and “A Conformable Electronic Ink Display using a Foil-Based a-Si TFT Array, SID01, DIGEST pp. 157–159”.
The above display device using the electronic ink is based on a simple principle and requires no illumination means that is used in the liquid crystal display devices and is, hence, called electronic paper, and has been studied concerning its widespread use to substitute for the conventional data transmission or recording means using papers as represented by newspapers and books.
In order for the electronic paper to be widely used in the society, means is necessary for supplying a display medium of large areas that can be folded like a newspaper at high speeds, in large amounts and at low prices just like printing characters and photographs on a paper. However, though the TFTs using the inorganic semiconductor layer can be produced based on an established technology maintaining stability, it is still necessary to form a semiconductor layer that serves as an active layer and to form a metal film for forming wirings and electrodes by utilizing vacuum like in the conventional semiconductor process, thereby to form a pattern through the photolithography
Further, in order for the users to enjoy the convenience of handling, it is not desired to use, as a substrate, brittle materials such as a glass, heavy and poorly flexible materials such as a stainless steel. A flexibility can be maintained to some extent when a thin plate such as of a stainless steel or aluminum is used as a substrate making, however, it is difficult to maintain flatness of the plate surface due to plastic deformation.